1. Field of the Invention
This invention relates generally to electrical contacts for maintaining electrical continuity between members subject to relative horizontal motion. More particularly, it relates to a contact arrangement for maintaining electrical continuity between upper and lower spaced surfaces, one of which is movable horizontally with respect to the other, while imposing minimal drag. This invention was made as a result of a contract with the United States Department of Energy.
2. Problem
This invention was made in response to a problem which arose during testing of an experimental system for automatically positioning a diamond-tipped cutting tool to a precision of .+-. 0.6 microinch. The cutting tool was mounted on a carriage which in turn was supported and guided along an axis by fluid bearings. The carriage was to be driven by a commercially available d.c. linear electric motor including a permanent magnet which was affixed to the carriage and which extended freely about an elongated horizontally extending stationary armature containing a series of axially spaced, separate windings. Mounted on the bottom of the armature was a horizontally extending linear commutator whose segments were connected to the windings. The permanent magnet carried a plurality of standard spring-loaded carbon brushes for contacting the commutator segments and selectively energizing the armature windings so that, for any position of the carriage, only the winding nearest the magnet was energized. The linear motor was connected in a conventional continuous-path closed-loop position-control system generally similar to that disclosed in U.S. Pat. No. 4,221,995, issued on Sept. 9, 1980, to W. E. Barkman. Unfortunately, tests of the experimental positioning system showed that because of excessive friction resulting in stick-slip problems, the carriage could be positioned with a precision of only about 35 microinches.
3. Previous Related Work
The above-referenced patent to Barkman discloses a positioning system in which the carriage is driven by a linear electric motor which does not utilize brushes or a commutator. The motor is relatively bulky and generates appreciable heat. U.S. Pat. No. 4,171,496, to Eriksson, describes an electrical contacting arrangement including two conductors which are disposed for relative rotation and are each formed with an annular ring. The rings extend respectively into electrically interconnected grooves provided in an auxiliary rotor. The rotor is driven to force liquid metal into the grooves, thus connecting the rings on the two conductors. U.S. Pat. No. 3,870,914, to Walker, describes an electrical contact arrangement for a vertically disposed rotatable electric machine in which mercury is vaporized from a heated reservoir in the base of the machine and condenses in a vertically extending capillary formed by a fixed contact ring and a contact affixed to the rotor of the machine. The machine volume exposed to mercury vapor is sealed from atmosphere. U.S. Pat. No. 3,916,235, to Massar, describes a contact arrangement in which two relatively rotatable conductors form an annular gap. Liquid metal from an external supply is pumped through bores to completely fill the gap and thus connect the conductors. Preferably, a metallic ring is provided in the annular gap to float on the liquid metal. U.S. Pat. No. 2,845,554, to Schwab et al, describes a contact arrangement for interconnecting a rotatable member and two spaced, fixed members. The rotatable member is formed with a ring of triangular section, which rotates in the annular space between the fixed members. An electrically conductive liquid is circulated through the annular clearances between the fixed and rotating members, through a cooler, and back to the clearances. None of these arrangements is well suited for use with upper and lower elongated members which are subject to relative horizontal movement.